Rocket motor with blast tube and case bonded propellant

ABSTRACT

2. A multiple end-burning rocket comprising a motor casing closed at its forward end and having rearward nozzle means, at least one propellant charge therein case-bonded to said motor casing and having a plurality of end-burning surfaces, and a hollow blast tube extending through said propellant charge and to which said charge is bonded, said tube being expansible and contractible in response to changes in the dimensions of said propellant charge, and said blast tube comprising a cured mixture of a liquid bisphenol epoxide polymer and a liquid polythiopolymercaptan polymer.

United States atet [191 Ritchey et al.

[451 out 16,1973

[ ROCKET MOTOR WITH BLAST TUBE AND CASE BONDED PROPELILANT [75]Inventors: Harold W. Ritchey, Huntsville, Ala; John M. McDei-mott,Brigham City,

Utah

[73] Assignee: Thiokol Chemical Corporation,

Bristol, Pa.

[22] Filed: Dec. 30, 1959 [21] Appl. No.: 857,642

[52] US. Cl 60/253, 86/1, 102/103, 60/219, 264/3 [51] Int. Cl. K 02i9/04 [58] Field of Search 60/356, 253;

[56] References Cited UNITED STATES PATENTS 2,937,595 5/1960 Margulis etal. 60/356 2,972,859 2/1961 Porcher 2,816,721 12/1957 Taylor 60/35.6

OTHER PUBLICATIONS 1958 Missle Materials Review by A. J. Zaehringer andR. M, Nolan, Published in Missiles and Rockets,

Mar. 1958, Vol. 3, No. 3, pp. 69-75.

Solid Fuels Contend for Long Range Ballistic Role by l. S. Rutz, Jr.,Aviation Week, Oct. 7, 1957. Pages 50, 51 and 52.

A Quasi-Morphological Approach to the Geometry of Charges for SolidlPropellant Rockets: The Family Tree of Charge Designs by J. M. Vogel,Jet Propulsion, Vol. 26, No. 2, pages 102-105, Feb. 1956.

Primary Examiner-Samuel lFeinberg Attorney-Gordon K. Lister EXEMIPLARYCLAIM 2. A multiple end-burning rocket comprising a motor casing closedat its forward end and having rearward nozzle means, at least onepropellant charge therein case-bonded to said motor casing and having aplurality of end-burning surfaces, and a hollow blast tube extendingthrough said propellant charge and to which said charge is bonded, saidtube being expansible and contractible in response to changes in thedimensions of said propellant charge, and said blast tube comprising acured mixture of a liquid bisphenol epoxide polymer and a liquidpolythiopolymercaptan polymer.

5 Claims, 3 Drawing Figures ROCKET MOTOR WITH BLAST TUBE AND CASE BONDEDPROPELLANT This invention relates to solid propellant rocket motors, andrelates in particular to solid propellant rocket motors in which thepropellant burns simultaneously at a plurality of end surfaces, and tomethods of making such rocket motors.

Solid propellant rocket motors commonly comprise a partially closedcylindrical motor casing having a propellant charge therein. Thepropellant charge is preferably bonded to the walls of the motor casing.Case bonding is conveniently accomplished by filling the casing with afluid curable propellant composition and curing in situ. As known in theart, such propellant compositions commonly comprise a combustibleorganic material capable of polymerizing, for example materials of thepolyurethane, polyester, or polyether types, and containing 65-95 partsby weight of a solid oxidizing agent mixed therewith. At one end thecasing is provided with a nozzle through which hot gases evolved bycombustion of the solid propellant pass to produce the motive impulsewhich drives the rocket. The configuration of the solid propellantcharge is an important variable affecting the amount of thrust generatedby the rocket.

To obtain high thrust values, a longitudinal internal burningconfiguration has commonly been used for rocket propellant charges. Ifthe propellant composition has a high burning rate, however, such acharge configuration may not allow a long enough burning time. The useof a slower burning propellant, on the other hand, limits the thrustavailable at a given time during the sustainer period of motoroperation. Consequently it has proved desirable to find a way to achieveboth high thrust and longer burning time when using a propellant havinga high burning rate.

A transverse or end-burning propellant charge has been suggested toachieve this end. However, such charges have been more the exceptionthan the rule in rocket motor technology, because the relatively smallcross-sectional area of the burning charge limits the thrust obtainable.I-Ieretofore, high thrust values have not been feasible using anend-burning propellant charge without an undesirable increase in thecrosssectional area of the charge and hence also the diameter of themotor casing containing the charge.

However, the burning area of a propellant charge in an end-burning motorcan be increased, without enlarging the diameter of the motor casing, bysimultaneously burning more than a single end surface of the propellantcharge. Such a procedure encounters certain technical difficulties. Forexample, to increase the thrust of an end-burning propellant charge bycausing burning to take place at both the forward and the aft end of thecharge, combustion products from the forward end-burning surfaces of thecharge must be conducted to the nozzle or aft end of the rocket motor bya duct extending through or around the propellant charge. A peripheralduct presents serious difficulties in supporting the charge inside themotor casing, and also requires insulation of both the charge and theeasing. The charge must be insulated because hot gases traveling past itat high velocity from the forward end of the motor are apt to causeignition and burning of the propellant along the sides of the charge,thereby interfering with the intended program of thrust and burningtime. Insulation of the motor casing is also required to protect thecasing from the eroding effects of the hot gases from the forward end ofthe burning propellant charge.

The use of a central duct in a multiple end burning rocket overcomesmany of the difficulties inherent in the use ofa peripheral duct. Such acentral duct or tube extending through the center of a propellant chargeconducts combustion products from forward burning surfaces back to thenozzle of a rocket holding the charge.

The present invention concerns a multiple endburning rocket having acentral, heat resistant duct or blast tube for conveying gases fromforward burning surfaces of a propellant charge through intermediateportions of the charge. These gases are led into a rearward chamber ofthe rocket and eject through the nozzle of the rocket. Gases generatedin forward portions of the rocket thereby contribute to the thrustprovided by gases simultaneously evolved at rearward burning surfaces ofthe propellant charge.

A better understanding of the invention and an appreciation of its manyadvantages may be had by reference to the accompanying drawings. Inthese drawings,

FIG. 1 is a side elevation in longitudinal section of a doubleend-burning rocket motor;

lFllG. 2 is a front elevation of the same motor in transverse sectiontaken along the line 2-2 of FIG. ll;

FIG. 3 is a side elevation in longitudinal section of another embodimentof a rocket motor, in which embodiment the propellant charge has fourtransverse burning surfaces.

FlGS. 11-3 show rockets comprising motor casing lll, advantageously ofcylindrical shape, forward closure l2, and nozzle means 13. Withincasing llll is an insulating liner ll4l of a refractory material. Inmore interior portions of casing lll is propellant charge 15, which maybe of a variety of solid propellant compositions known to the art suchas a combustible organic resin, e.g. polystyrene, mixed with anoxidizing agent, e.g. ammonium perchlorate. lPropellant charge l5surrounds centrally mounted duct llo, which extends from the forward tothe aft end of the charge. In the rockets of FIGS. l and 3, propellantcharge 15 has forward and aft burning surfaces 117 and 11%. In addition,charge R5 of the rocket of FIG. 3 has in its middle portions two moreburning surfaces H9. Hot gases from burning surfaces l7 and H9 areconducted through hollow interior 20 of duct M to nozzle portions 113 ofthe rockets. The propellant charge is initially ignited at the surfacesto be burned using conventional igniter means (not shown in thedrawings), for example either of the chemical or electrical types.

In conducting the hot gases from the forward burning portions of thepropellant charge through the central duct or blast tube to the rearwardnozzle, ignition and burning of that part of the propellant chargebetween the multiple burning ends is to be avoided. If the intermediateportion of propellant charge does ignite and burn, a rapid rise in thetotal pressure of the combustion gases can be caused. Such a rapid risein pressure may be sufficient to burst the motor casing. Also, if anypart of the propellant charge except the intended burning surfaces areignited, the planned program of thrust and burning time for the motormay be altered.

To avoid ignition of intermediate portions of the propellant charge, thecentral duct or blast tube may be made of ceramic, or of metal, or ofsome other such nonflammable and relatively heat-resistant material.However, blast tubes of this type are relatively heavy and contributeextra weight to the rocket. Also, in the process of bonding thepropellant charge to the motor casing, the propellant may tend to shrinkaway from the blast tube during the curing period. This phenomenon maybe encountered if the blast tube is made of a material of high elasticmodulus. Furthermore, as the motor is heated up by the combustion takingplace during firing, temperature stresses may be set up between suchmetallic or ceramic blast tubes and the propellant charge. Cracking ofthe propellant charge and subsequent uneven burning or runawayconditions may ensue.

It has been discovered that the use of a light weight, heat resistant,synthetic resinous material in the blast tube is to be preferred. A tubeof this type obviates many of these difficulties attendent upon the useof ceramic or metal. The synthetic resinous materials herein consideredfor use in a blast tube have a lower density than suitable metals, andalso possess those physical properties which allow expansion andcontraction of such a tube with a propellant charge in contact with it,so that a uniform, void-free contact between the tube and charge isobtained. The blast tube materials can expand or contract with thepropellant charge, permitting accommodation to temperature changes overa wide range. The synthetic resinous compositions have a low thermalconductivity and can provide a much more effective barrier to heat flowthan can a refractory type material such as metal or ceramic. The resincompositions decompose pyrolytically in an endothermic reaction atmoderate temperatures to leave a coherent ash of high melting point andlow thermal conductivity] For example, a heat resistant, syntheticresinous composition of this type is one made from a cured mixture of aliquid polyepoxy compound and a liquid organic polysulfide polymer withan inorganic refractory filler. Such compositions advantageously containbetween about 50 parts by weight to about 90 parts by weight of thepolyepoxy compound and between about 50 parts and about parts by weightof polysulfide polymer. The filler content is advantageously betweenabout 30 per cent by weight to about 45 per cent by weight of the totalmixture, which is cured with about 10 per cent by weight of an alkalinecuring agent, preferably an amine or polyamine, primary, secondary ortertiary.

The epoxy materials preferred for use in the manufacture of theexpansible and contractible blast tubes are monomeric or partiallypolymerized polyglycidyl ethers produced by the reaction of an epoxycompound, such as epichlorohydrin, with a polyglycol or polyphenol, suchas glycerine of the dihydroxydiphenylmethanes. Materials of this type,which are commonly prepared by reacting two or more molar proportions ofepichlorohydrin with one molar proportion of a polyglycol or polyphenolin the presence of an alkali to neutralize hydrochloric acid produced inthe reaction, are described, for example, in U.S. Pat. No. 2,506,486issued May 2, 1950 to Bender, Farnham and Guyer. Products such as theseare commerically available under such tradenames as Araldite 504," madeby the ClBA Company, or the Epon" resins such as Epon 828," Epon 562made by the Shell Chemical Corporation, Epirez 510, a product of theJones- Dabney Company, ERL 2794, a product of Union Carbide and CarbonCompany, etc.

The liquid organic polysulfide polymers are, for example, thosedisclosed in U.S. Pat. No. 2,466,963 to Patrick et al., and may beconveniently described as polythiopolymercaptan liquid polymers. Arepresentative polymer of this type is marketed by the Thiokol ChemicalCorporation under the name Thiokol" LP33.

Numerous inorganic fillers can be added to the resin compositions,including mineral oxides, glass fibers, carbon black, graphite, andasbestos, for example. Asbestos is a preferred material and isadvantageously incorporated into these compositions in the form of f1-bers, and preferably as S-floats.

A preferred composition of the type described is given below:

Ingredient Parts by Weight Liquid polyepoxy compound (Araldite 504") 33Liquid polythiopolymercaptan polymer (Thiokol" LP33) 22 Inorganic filler(Asbestos S-floats) 42 Amine catalyst (2,4,6-tridimethylaminomethylphenol) 3.3

The preferred blast tubes are prepared by casting mixtures of the kinddescirbed in appropriate molds and letting the mixtures harden. Theblast tubes are then positioned within a rocket casing and a casebonding propellant composition cast thereabout. When blast tubes orducts of the expansible and contractible materials taught are used, thedifficulties of bonding to the tubes, often experienced when metal orceramic tubes are employed, are obviated.

Upon ignition of the rocket, blast tubes of the preferred expansible andcontractible materials freely conduct the hot gases generated fromforward burning portions of the propellant charge to posterior portions.The compositions, which are believed to char in surface portions leavinga refractory ash, effectively insulate intermediate portions of thepropellant charge which surround the tube so that no ignition of thesenon-surface portions of the charge occurs. Despite extreme temperaturedifferentials, the blast tubes made from the synthetic polymers remainbonded to the propellant charge, accommodating to volume changes in thelatter.

Although specific embodiments have been herein shown and described, itis to be understood that they are illustrative, and are not to beconstrued as limiting on the scope and spirit of the invention.

What is claimed is:

l. A multiple end-burning rocket comprising a motor casing, closed atits forward end and having rearward nozzle means, a propellant chargetherein bonded to said motor casing and having a plurality of endburning surfaces, and a hollow heat resistant elastic central duct insaid propellant charge for conducting gases from anterior to posteriorportions of said rocket, said charge being bonded to said duct, and saidduct comprising a cured composition comprising between about 50 partsand part by weight of a liquid polyepoxy compound, between about 50parts and 10 parts by weight of a liquid polythiopolymercaptan polymer,about 10 per cent by weight of 2,4,6-tri-dimethylaminomethyl phenolbased on the weight of epoxide resin, and between about 30 per cent and45 per cent by weight of asbestos filler, based on the total weight ofthe mixture.

2. A multiple end-burning rocket comprising a motor casing closed at itsforward end and having rearward nozzle means, at least one propellantcharge therein case-bonded to said motor casing and having a pluralityof end-burning surfaces, and a hollow blast tube extending through saidpropellant charge and to which said charge is bonded, said tube beingexpansible and contractible in response to changes in the dimensions ofsaid propellant charge, and said blast tube comprising a cured mixtureof a liquid bisphenol epoxide polymer and a liquid polythiopolymercaptanpolymer.

3. A rocket as in claim 2 wherein said cured mixture additionallycomprises a refractory solid mixed with

1. A multiple end-burning rocket comprising a motor casing, closed atits forward end and having rearward nozzle means, a propellant chargetherein bonded to said motor casing and having a plurality of endburning surfaces, and a hollow heat resistant elastic central duct insaid propellant charge for conducting gases from anterior to posteriorportions of said rocket, said charge being bonded to said duct, and saidduct comprising a cured composition comprising between about 50 partsand 90 part by weight of a liquid polyepoxy compound, between about 50parts and 10 parts by weight of a liquid polythiopolymercaptan polymer,about 10 per cent by weight of 2,4,6-tri-dimethylaminomethyl phenolbased on the weight of epoxide resin, and between about 30 per cent and45 per cent by weight of asbestos filler, based on the total weight ofthe mixture.
 2. MULTIPLE END-BURNING ROCKET COMPRISING A MOTOR CASINGCLOSED AT ITS FORWARD END AND HAVING REARWARD NOZZLE MEANS, AT LEAST ONEPROPELLANT CHARGE THEREIN CASE-BONDED TO SAID MOTOR CASING AND HAVING APLURALITY OF END-BURNING SURFACES, AND A HOLLOW BLAST TUBE EXTENDINGTHROUGH SAID PROPELLANT CHARGE AND TO WHICH SAID CHARGE IS BONDED, SAIDTUBE BEING EXPANSIBLE AND CONTRACTIBLE IN RESPONSE TO CHANGES IN THEDIMENSIONS OF SAID PROPELLANT CHARGE, AND SAID BLAST TUBE COMPRISING ACURED MIXTURE OF A LIQUID BISPHENOL EPOXIDE POLYMER AND A LIQUIDPOLYTHIOPOLYMERCAPTAN POLYMER.
 3. A rocket as in claim 2 wherein saidcured mixture additionally comprises a refractory solid mixed with saidpolymers.
 4. The method of making a multiple end-burning rocket motorwhich comprises centrally positioning a hollow duct of a cast heatresistant synthetic resin within a motor casing, said resin comprising abisphenol epoxide polymer and a polythiopolymercaptan polymer, andcasting thereabout a case-bonding propellant charge having anterior andposterior end-burning surfaces, said duct being expansible andcontractible in response to changes in the dimensions of said propellantcharge.
 5. The method as in claim 4 wherein said resin additionallycomprises a refractory solid mixed with said polymers.